The citywide speed limit on local streets drops to 25 mph on Friday — ­although Mayor Bill de Blasio promised to ease into the new law with public education and warnings before any ticket blitzes begin.

Enforcement will rise as more drivers learn about the new limit, officials said.

“It will be phased in,” said de Blasio. “We will be doing a lot of public education over the coming months. We’ll be doing a lot of warnings — obviously, enforcement over time.”

The new law formally went into effect at midnight, and the Department of Transportation said installation of 25-mph signs would be under way during the morning commute.

The signs, made at a DOT sign shop in Maspeth, Queens, will replace 3,000 30-mph signs around the five boroughs.

Among other locations, they will go up along roads at city borders, as well as near airport car-rental agencies. The new signs will also be placed at highway exits and on streets with high crash rates.

Cyclists must also follow the 25-mph law, authorities said.

Highway speed limits will remain the same, and some larger thoroughfares will stay at 30 mph for now.

Officials said recently that cops can use discretion and focus on drivers traveling significantly over the limit.

Summonses for drivers going up to 10 mph above the limit range from $45 to $150, according to the Department of Motor Vehicles.

Motorists who go between 10 and 30 mph over the speed limit face fines ranging from $90 to $300 — as well as jail time up to 15 days.

New signs ready for installation at the DOT’s Maspeth sign shopJames Messerschmidt

And speed demons caught going 30 mph and up above the limit pay between $180 and $600, and can be thrown in jail for up to a month.

“If the NYPD enforces it properly and consistently, the new speed limit can help the city cut the number of yearly pedestrian ­fatalities in half, and prevent thousands of injuries,” said Paul Steely White of Transportation Alternatives.

“Speeding kills more New Yorkers than drunk driving and cellphone use at the wheel put together.”

DOT employees spread out Thursday around town to alert drivers to the new limit, at intersections such as in Midtown Manhattan, Bay Ridge, Brooklyn, and Jackson Heights, Queens.

The lower speed limit is part of de Blasio’s Vision Zero plan to end traffic deaths in the city.

People struck by vehicles at 25 mph are 50 percent less likely to die than at 30 mph, statistics show.